Accelerating ocean warming revealed by extensive oceanic buoy network

Global warming is driven by Earth’s energy imbalance, the difference between the solar energy absorbed by the Earth and the energy radiated by Earth into space. Since most of the energy imbalance is stored in the oceans, measuring changes in ocean heat content provide the most reliable estimate of the planetary energy imbalance. Improved data coverage and analysis has made it possible to reconstruct temperature profiles across most ocean basins and at all depths to 6000 meters (m) from 1960 to 2015. The reconstructions reveal accelerating heating in the upper layers above 2000 m. Ocean warming is stronger since the late 1980s compared to the 1960s to the 1980s. The rate of warming in the deep ocean below 2000 m has been relatively slow and the heat uptake is only 13% of the estimated total oceanic heat uptake rate between 1993 and 2010. This analysis confirms that most of the Earth’s energy imbalance has been stored in the upper ocean layers above 2000 meters.

Introduction

Earth’s energy imbalance (EEI) is the difference between the energy received from the Sun and the energy emitted by the Earth back to space. The magnitude of the imbalance is the fundamental metric determining the rate of climate change. Since the Earth’s oceans absorb 93% of the planetary energy imbalance, with the remaining 7% accounted for by land warming, atmosphere warming and moisture gain, and melting ice, tracking the change in ocean heat content (OHC) over time provides the most reliable estimate the energy imbalance.

Observations: Ocean heat content 0 – 2000 m

Estimating ocean heat content requires recording many temperature profiles from the surface to the ocean floor across the world’s oceans. Early observations of ocean temperatures were carried out on commercial and scientific research vessels typically near developed countries and along shipping routes. From 1990 to 1997 the World Ocean Circulation Experiment extended these measurements to a global network.

Argo is a global array of 4000 free-drifting profiling floats that measures the temperature and salinity of the upper 2000 m of the ocean. Status March 2017

Since about 2005 Argo floats (reporting temperature profiles every 10 days) have been deployed across the oceans and have dramatically increased data coverage, with the exception of coastal, marginal sea, deep ocean and ice-covered regions. But now Argo floats are being deployed even to those areas of the oceans.

The challenge in estimating ocean heat content is to find a method for filling the data gaps in both space and time while minimizing sampling error. One way of doing this is to statistically fill data gaps by using available observations and an adjustment term based on nearby observations. A complete gridded temperature field for 0- to 2000-m depths from 1960 to 2015 has been used as the basis for a reconstruction of temperature across all ocean basins. Using this reconstruction, an updated historical (1960–2015) ocean energy budget and contributions to Earth’s total energy budget were calculated. When the global ocean is divided into a monthly 1°-by-1° grid, the monthly data coverage is 10% before 1960, 20% from 1960 to 2003, and 30% from 2004 to 2015.

Observations: Ocean heat content below 2000 m

Most of the data relating to changes in ocean heat content are restricted to the upper 2000 m. There are challenges in sampling the deeper reaches of the ocean. In a recent study ocean warming for depths greater than 2000 m have been investigated using 35 years of hydrographic survey data from 1981 to 2015. The data is analyzed to reconstruct temperature changes over decades at deep (2000-4000 m) and abyssal (4000-6000 m) levels.

The data is comprised of high-quality deep ocean temperature, salinity, and pressure data publicly available as of April 2016 at a public data sitepublic data site. A conductivity-temperature-depth (CTD) instrument was used to measure temperature, salinity, and pressure. Along the cruise tracks samples were taken every 55 km at multiple depths. The global data set consists of 148 hydrographic sections repeated along 34 cruise tracks spanning 1981 to 2016, The area of each basin at each pressure level is used as a weight to compute five distinct temperature trend profiles for the Atlantic Ocean, Pacific Ocean, Indian Ocean, Southern Ocean, and global ocean.

Changes in ocean heat content

Global ocean heat content. (A) OHC from 0 to 700 m (blue), 700 to 2000 m (red), and 0 to 2000 m (dark gray) from 1955 to 2015. The uncertainty is shown in shading. (B) The new estimate is compared with an independent estimate from NCEI. Both OHC 0 to 700 m and OHC 700 to 2000 m are shown from 1957 to 2014. SOURCE: Cheng at al.

On the basis of the reconstructed ocean temperatures monthly changes in ocean heat content within the 0 to 700 and 700 to 2000 m layers have been calculated.

These reveal significant warming in the past 56 years. Ocean warming is stronger since the late 1980s compared to the 1960s to the 1980s for both 0 to 700 m and 700 to 2000 m depths.

Depth

1960–1991

1992–2015

Acceleration

0-700 m

0.15 × 10**22 J/year

0.61 × 10**22 J/year

4X

700-2000 m

0.04 × 10**22 J/year

0.37 × 10**22 J/year

9X

The analysis revealed an accelerating heat input into both the 0 to 700 m and 700 to 2000 m layers. In the upper reaches of the ocean the warming trend was four times stronger during 1992-2015 than during the earlier period. For depths of 700-2000 m the warming trend was nine times stronger than that from 1960–1991.

Of the total increase in ocean heat content from 1998 to 2015 in the upper 2000 m, 17% of the heat was stored in the Pacific Ocean, 24% in the Indian Ocean (30°S northward), 31% in the Atlantic Ocean, and 28% in the southern oceans (south of 30°S).

Results below 2000 m

Mean temperature trends (in thousandths °C per year) for the global ocean for the long-term estimate (black), pre-2000 (blue) and post-2000 (orange), and the difference (green) between the pre- and post-2000 profiles. The 95% confidence intervals are shown shaded. SOURCE: Desbruyères et al.

The long-term global temperature trend for the period 1991–2010 reveals warming at all levels below 2000 m. The trend values are statistically significant at every depth and the uncertainty decreases with depth. The strongest warming rates are found in the abyssal layer (4000–6000 m).

Depth

Thousandths of °C per yr

Proportion of total heat gain below 2000 m

Deep 2000-4000 m

0.34 ± 0.19

67%

Abyssal 4000-6000m

0.53 ± 0.11

33%

Average

0.39 ± 0.17

100%

Comparing warming rates before the year 2000 and after 2000 revealed that the global average warming rate in the deep ocean did not change. The heat uptake below 2000 m during this period is only 13% of the estimated total oceanic heat uptake rate between 1993 and 2010.

Ocean energy budget

Estimate of the ocean energy budget relative to the 1958–1962 base period. Satellite measurements of net energy imbalance are shown in yellow. The dashed gray lines are the 95% confidence interval. The three major volcanic eruptions are marked. SOURCE: Cheng at al.

The ocean energy budget shown in the figure is based on the reconstruction of ocean heat content by Cheng at al. in the upper 0 to 2000 m and the reconstruction below 2000 m from Desbruyères et al.

The latest reconstruction provides an estimate of total full-depth ocean warming of 33.5 × 10**22 joules (J) from 1960 to 2015. (It requires 560 × 10**22 J to raise the temperature of the oceans by 1 °C.) This amount of incremental energy evenly distributed over all ocean basins and depths would raise the average temperature of the oceans by o.o6 °C. But this heat is unevenly distributed over the layers of the ocean with 87 % in the upper ocean above 2000 m.

Depth

proportion of total heat gain

0- to 300-m

36.5 %

300- to 700-m

20.4 %

700- to 2000-m

30.3 %

below 2000-m

12.8%

This new reconstruction indicates that the vast majority of the ocean warming since 1960 has been in the top 0 to 2000 m layer.

Comparison of the Argo data with previous temperature data collected by HMS Challenger (1872–1876) suggests that the temperature change since the 1970s is 0.33 °C average increase in the upper portions of the ocean to 700 meters depth. This increase ranges from 0.59 °C at the surface to 0.12 °C at 900 meters depth.

Consistent with satellite radiation measurements

Changes in ocean heat content provide the most reliable estimate of the Earth’s energy imbalance. Radiation measurements from space offers another way of estimating the energy imbalance. The yellow curve in the figure above shows the energy imbalance computed from satellite radiation measurements with the assumption that 93% of the imbalance is absorbed by the oceans. Comparison of the energy imbalance based on measured changes in ocean heat content and satellite radiation measurements shows consistent long-term changes since the late 1980s. This further confirms this new assessment of Earth’s ocean energy inventory.

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